PROJECT SUMMARY Immunotherapies, and specifically immune-checkpoint inhibitors, are revolutionizing the therapeutic paradigm for several solid tumors. To date, however, the response to these drugs in patients with castration resistance prostate cancer (CRPC) has been modest, despite that a vaccine therapy, sipuleucel-T, has been approved based on survival data. Several hypotheses have been proposed to explain this intrinsic resistance, including an immunosuppressive prostate tumor microenvironment. Diet modifications have been reported to impact tumorigenesis and tumor progression, including prostate cancer models. Dietary restriction of methionine has been shown to extend the lifespan of rodents, to reduce inflammation in healthy animals and to enhance metabolic activity. Sulfur containing amino acids such as methionine and cystine are precursors for thiols and critical for several cellular processes including immune system. Results from our lab have shown that dietary protein restriction significantly decreases tumor growth in preclinical models of castration resistant prostate cancer. The inhibition of tumor growth is associated with reduction of tumor infiltrating macrophages with M2, immunosuppressive phenotype. Our preliminary data also show that methionine restriction induces in vitro polarization of bone marrow-derived macrophages to a M1, immunopromoting phenotype. Thus, there is a rationale to determine the role of dietary protein modification and, in particular methionine restriction, on modulating the immune response in prostate cancer. Our central hypothesis is that reduction in dietary protein intake may have a significant impact on prostate cancer by modifying the immunosuppressive microenvironment and enhancing innate and adaptive immune response. The principal objective of this proposal is to exploit the role of diet intervention in the treatment of prostate cancer in the setting of immunotherapies. Specific Aims: To achieve our goal we will pursue the following specific aims: 1) To assess the impact of dietary protein or methionine restriction on the prostate tumor microenvironment both in vitro and in vivo; 2) To determine the impact of protein diet modification or methionine restriction on prostate tumor response to immunotherapies The proposed study is highly innovative by introducing diet modifications as an adjuvant treatment in in prostate cancer. Our proposed studies will determine how altered amino acid diets may enhance the response to immunotherapeutic treatments. This will contribute to the scientific field by advancing knowledge of diet and CaP/cancer in general with a focus of advancing immunotherapeutic treatments. Translation of this research may provide novel less invasive and more appealing adjuvant treatments for CaP patients.